Vietnamese air force planes on Saturday spotted two large oil slicks close to where a Malaysia Airlines Boeing 777 went missing earlier in the day, the first sign that the aircraft carrying 239 people had crashed.

The air force planes were part of a multinational search operation launched after Flight MH370 fell off radar screens less than an hour after it took off from Kuala Lumpur for Beijing early Saturday morning.

The oil slicks were spotted late Saturday off the southern tip of Vietnam and were each between 10 kilometers (6 miles) and 15 kilometers (9 miles) long, the Vietnamese government said in a statement. There was no confirmation that the slicks were related to the missing plane, but the statement said they were consistent with the kinds that would be produced by the two fuel tanks of a crashed jetliner.

Two-thirds of the missing plane's passengers were from China, while others were from elsewhere in Asia, North America and Europe.

Malaysia Airlines CEO Ahmad Jauhari Yahya said there was no indication that the pilots had sent a distress signal, suggesting that whatever happened to the plane occurred quickly and possibly catastrophically.

At Beijing's airport, authorities posted a notice asking relatives and friends of passengers to gather at a nearby hotel to wait for further information, and provided a shuttle bus service. A woman wept aboard the bus while saying on a mobile phone, ''They want us to go to the hotel. It cannot be good.''

Relatives and friends of passengers were escorted into a private area at the hotel, but reporters were kept away. A man in a gray hooded sweatshirt later stormed out complaining about a lack of information. The man, who said he was a Beijing resident but declined to give his name, said he was anxious because his mother was on board the flight with a group of 10 tourists.

''We have been waiting for hours and there is still no verification,'' he said.

The plane was last detected on radar at 1:30 a.m. (1730 GMT Friday) around where the South China Sea meets the Gulf of Thailand, authorities in Malaysia and Vietnam said.

Lai Xuan Thanh, director of Vietnam's civil aviation authority, said air traffic officials in the country never made contact with the plane.

The plane ''lost all contact and radar signal one minute before it entered Vietnam's air traffic control,'' Lt. Gen. Vo